Coming so close to Phobos without taking photos may sound like forgetting to pack your camera before an expedition to the Serengeti, but as the flyby will be so low and so fast, any attempt at photography would look like a blurred mess. So the flyby will instead be used as a prime opportunity to accurately measure Phobos' gravitational field.

The Dec. 29 encounter with Phobos will slightly pull Mars Express off course, changing the satellite's velocity. The European Space Agency will be closely monitoring radio communications with the spacecraft so they can accurately measure the slight deviation. This, in turn, will reveal the gravitational oomph of the moon, allowing scientists to precisely deduce its mass and density.

In March 2010, Mars Express carried out a 67 kilometer flyby of Phobos, revealing the moon could be up to one-third empty space. This has led to the idea that Phobos is likely a "rubble pile" — an agglomeration of smaller pieces of debris that have collected under mutual gravity.

It is thought that Phobos, and its smaller sibling Deimos, are either captured asteroids that strayed into Mars' gravitational field or the debris from an ancient catastrophic impact event. Through precision measurements of the moon's gravitational field further clues as to the origin of Mars' natural satellites may be gleaned.

Also, as the satellite will be swooping so close to Phobos' dusty surface, a measure on the solar wind's interactions with its surface can also be acquired.

Interestingly, the flyby comes a decade after Mars Express arrived at Mars orbit.

"Mars Express entered orbit around the Red Planet exactly ten years ago this week — this close flyby of Phobos is certainly an exciting way to celebrate!" said Olivier Witasse, ESA's Mars Express project scientist.